Said it before, will say it again. Probably will be saying it for eternity: Gaston should've been in Days. Days had Beast's Castle presumedly to focus to Xaldin's 1st meeting with Beast. which we didn't even see! WHY does the Beast listen to a single damn word Xaldin ever says to him to begin with? The easiest explanation would be if Xaldin had saved Beast's life, thus earning some trust.
Have Gaston as the boss, playing as Roxas with Xaldin as party member. In the cutscene after the fight Xaldin deals the fatal blow & then we get the most obvious how-could-they-not-think-of-that pun ever:
Gaston: Nobody can defeat Gaston!
*slice*
*falls to his death*
Xaldin: "Nobody" did.
For extra evil bonus points, have it that Xaldin was the one who tipped Gaston off about the Beast holding Belle captive in the 1st place. Thus Xaldin being there at the right time to save Beast would be no mere coincidence.